1. Technical Field
The present invention relates to an apparatus and process for zinc-nickel electroplating.
2. Description of the Prior Art
U.S. Pat. No. 5,162,079 discloses an apparatus for electroplating metals. The apparatus comprises an electroplating bath which contains a plating solution of a metallic salt, for instance, nickel sulfate. A cathode workpiece is positioned in the bath. An insoluble anode assembly is also provided in the bath. The anode assembly includes an anode which is essentially insoluble during electroplating and an anion exchange membrane enclosure around the anode. An electrically conductive acid solution is contained within the enclosure of the anode assembly. The flow of current in the apparatus causes anions, for instance sulfate ions, in the plating solution to travel through the anion exchange membrane increasing the acid concentration within the anode assembly enclosure. Accumulated acid is periodically flushed from the enclosure. One purpose of the apparatus of the ""079 patent is to inhibit the increase in concentration of dissolved metal in the electroplating bath due to a cathode efficiency which is less than the anode efficiency.
U.S. Pat. No. 4,778,572 discloses an apparatus similar to that of the ""079 patent. An electroplating apparatus for plating nickel onto a workpiece is provided. A nickel-plating bath is provided in the apparatus. The bath is a typical Watts nickel low pH acid bath. A cathode workplece is positioned in the bath. An anode structure is also positioned in the bath. The anode structure comprises a series of nickel plate anodes. The nickel plate anodes are enclosed in an ion exchange membrane that allows a current flow from the anodes to the cathode workpiece while at the same time shielding the anodes from organics, such as Coumarin within the bath. The nickel plate anodes are immersed in dilute sulfuric acid contained within the ion exchange membrane enclosure.
German Patent Publication DE 19834353A1 published Feb.3, 2000, discloses an apparatus similar to that of the ""079 patent for applying a zinc-nickel coating onto a cathode workpiece. The apparatus comprises a vessel which is divided by a cation exchange membrane into a cathode compartment containing a catholyte and an anode compartment containing an anolyte. The catholyte is an alkaline zinc-nickel electroplating bath containing poly(alkyleneimine) additives for complexing and brightening. A cathode workpiece to be plated is positioned in the cathode compartment. The anolyte is an acid such as sulfuric acid or phosphoric acid. A platinum coated titanium anode is immersed in the anolyte. The ion exchange membrane allows the flow of current from the anode to the cathode, but at the same time shields the anode from the alkaline zinc-nickel electroplating bath.
Electrolysis of alkaline zinc-nickel baths containing poly(alkyleneimines) produces amine breakdown at the anode into nitriles and cyanides if the anode is exposed to the plating bath. The ion exchange membrane prevents such amine breakdown. However, an apparatus which comprises an alkaline electroplating bath adjacent to an acid anolyte can be dangerous. In addition, a platinum coated titanium anode is expensive.